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Disappearing graphics card problem

About a year and a half ago the graphics card in my old desktop stopped sending video to the monitor, but I could hear that Windows booted anyway. It was a problem that had gotten worse and worse over some time; I could sometimes wiggle the card in its slot and get it to work again, but not for very long and eventually nothing would make it work.
Since the problem could be the motherboard, the graphics card or even the PSU, I couldn't afford to guess my way to fixing it, so I used my laptop for a year until the fan broke on that and I had to emergency buy another desktop.

My parents have a desktop that is almost identical to my old one and about three weeks ago it stopped working. I guessed that it was the PSU and replaced theirs with the one from my old desktop and that worked fine, until yesterday.
Their computer had been slower than it should be for a long time, so since I'm staying in their house while they're on vacation, I thought I would check their HD to see if that might be the problem (there's always been way more HD traffic than I think there should be) and started a boot-time disk check.
After some hours the monitor went into powersaving mode and I couldn't get it to wake up again. The back of the computer near the graphics card felt very hot (which seemed odd because the white text on black screen during the disk check could hardly be very graphically taxing).

Long story short(er), after some reboots and waiting for a very long time until the disk check had finished, so the system would start booting into Windows instead, I still had no signal, but due to Logmein being on their computer, I can see that Windows is running and that there's apparently no graphics card anymore; Windows and CCleaner only show the Logmein driver and don't list a real card anywhere. GPU-Z just hangs while trying to get info.

So,
1.) Isn't it odd that the system boots if it thinks there's no graphics card? It does beep at boot and if I remember correctly, that code is something to do with graphics, but it still boots.
2.) Since this has happened on both my old system and my parents', when using the same PSU, doesn't it seem most likely that it's a problem with the PSU rather than the graphics cards? The card does generate a lot of heat for something that appears to not be doing anything, though.
It also seems odd that if the PSU is underpowered, it only affects the graphics card on both systems (I pulled everything noncritical from my old system to make it use as little power as possible, but that had no effect), but maybe that could be due to one rail being underpowered rather than everything. It does appear to be a rather cheap PSU.

PSUs can easily fry components - although they tend to do this by sending spikes of current which fries the component quickly rather than overheating it over time.

That's what puzzles me; when things went wrong in my computer, it was sort of partial. Sometimes I could wiggle the card and make it work again, for example. The last time I made it work, it lasted for over half a year before it finally went completely wrong. I actually thought the socket on the MB was the problem because it started to work when I inserted the card a bit wrongly, sort of pulled it to one side.

Mobo faults can also kill PSUs - I learned that the expensive way :)

Windows will happily boot without a GPU. Remember that many 'PCs' are headless - e.g. they're servers of some sort (actual servers or part of a cashpoint or whatever) so it has to.

yeah... hm, this makes me wonder if it was the MB that got fried in my old computer because it eventually stopped booting (a good while after it had stopped sending a video signal to the monitor).

It's entirely possible that your only problem is that your parents PSU was knackered and yours isn't much better - first job is to get another PSU - even a cheap one - and test that out.

I don't suppose it's possible to get an old pre-SATA, pre-PCI-E PSU to work with newer motherboards? Would make it cheaper to test.